BINGO!

Validating source code does nothing. Even setting aside the fact that firmware and hardware level exploits could be running in any given voting machine, what guarantee do we have that the code in the repository is the code actually running on the machine at the app layer? Or, if you're REALLY security minded like myself, what guarantee do we have that the compiler and build environment didn't slip something in? (I guess that borders more on "paranoid" than "security minded", but you get the idea.)

Unless you specify open testing protocols that also test for cheating (like VW emissions).

It shouldn't be this hard to vote and to trust a certified outcome. Regulators require businesses to run with specified parameters, why do elections get off the hook?

It's not hard, if you use paper ballots and well-established protocols for securing elections.

On the other hand, if you use electronic machines without paper backup, it's basically impossible.

Then why do electronic machines in the first place?

Because they're cheaper. I lived in counties that formerly used mechanical voting machines. They were replaced by electronic ones when the cost of maintaining them became higher than the replacement cost. If security was considered at all, I speculate that it was not the highest priority.

I agree but only if paper voting is abolished, otherwise you'll have both systems in place. Or is the paper only used when there are concerns about the results?

Exactly. They provide nothing over the security of a paper ballot.